Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers

Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called twist-bend nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)(7)CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d similar to 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q similar to 2 pi/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d similar to in CB(CH2) 7CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be 0(TB) similar to 25 degrees and the full pitch of the director helix to be p(TB) similar to 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend.